The present invention relates to a pneumatic tire suitable for heavy duty vehicles e.g. truck and bus, and more particularly, a tread pattern provided with shoulder ribss such as rib patterns, rib-block patterns and the like which can alleviate uneven wear.
In general, pneumatic tires having a carcass can be classfied into two types from the view point of carcass construction, that is, bias tires and radial tires. As to tread patterns, there are many types such as the rib type, the lug type, the block type, the rib-block type, the rib-lug type, the lug-block type and the like. And such carcass types and tread pattern types are combined in accordance with the use for the tire.
For example, in heavy duty radial tires for trucks and buses which run on paved roads, often used are rib type patterns formed by circumferentially extending ribs and rib-block type patterns formed by a pair of shoulder ribs extending circumferentially one at each tread edge portion and blocks in the crown region between the shoulder ribs.
However, tires having such types of tread patterns, tend to wear more rapidly in the tread shoulders than the tread crown. This kind of uneven wear is called shoulder wear, which wear is formed in such a way that the axially outermost edge of the shoulder rib wears partially due to cornering and so on, and the partial wear grows and extends unevenly in the circumferential direction and axial direction of the tire due to the difference in diameter between the tread crown and the tread shoulders. Also the slip during cornering, and consequently the amount of wear of the shoulder rib becomes larger than that of the tread element therebetween such as a rib and a block.
In order to prevent shoulder wear, as shown in FIG. 5, it has been proposed to form a circumferentially continuously extending straight narrow groove G on each shoulder rib SR. This narrow groove is effective to a certain extent for preventing the partial wear from growing axially inwardly of the tire. However, the axially outside part OR of the shoulder rib divided by the narrow groove G operates as an isolated narrow rib, and because of its narrow width, this outside part is moved easily to accelerated the growth of the wear.
On the other hand, in order to improve traction performance, wandering performance and the heat radiation power of tire, the tire is generally provided in each of its shoulder regions or tire shoulders with a buttress portion BA by forming buttress hollows H, the radially outer ends of which are opened at the tread surface T at intervals in the circumferential direction of the tire, as marked in FIG. 5 by chain lines.
When such buttress hollows are formed on the above-mentioned tire which is provided on each shoulder rib SR with the circumferentially continuously extending straight narrow groove G, the shoulder rib is decreased in rigidity at the hollowed portions IH between buttress hollows H and narrow groove G, and those portions IH is moved largely during running and, as a result, they become nucleuses of partial wear. Thus there exist a problem that the shoulder wear is induced as explained above.
It is, therefore, an object of the present invention to provide a pneumatic tire, which is provided with buttress hollows and narrow grooves on tread shoulder ribs for preventing the axially inward growth of partial wear, and in which the shoulder ribs are uniform in rigidity to lessen the partial movement thereof and to alleviate shoulder wear.